Plants’ Defenses Reveal Capacity to Fight Disease

André Velásquez, research associate in the MSU-DOE Plant Research Laboratory, and colleague Matt Oney wanted to find out how many types of defenses a plant could muster against harmful bacteria. No other study had ever done that on a large-scale; at most, a few tackled 20, 40, maybe 80 plant varieties at any one time.

So, Velasquez, Oney, and their mentor, Sheng Yang He, examined over 1000 natural variations of one plant – found in places ranging from Europe to Western Asia – and dipped them into pots full of harmful bacteria to see who would make it through unscathed.

And only a fraction ofVelasquez’s plants actually fought off the bacteria. “We found that these plants employed at least four types of previously reported defense mechanisms,” says Velasquez, a post-doc in the He lab. “They ranged from surface defenses that prevent an invasion to molecular defenses that are activated once the bacteria breach through. In some cases, we didn’t know how the defenses were working, which shows the limits of our current knowledge.”

“Plants, generally, don’t have an adaptive immune system like humans. If a strain of bacteria successfully infiltrates a plant once, the next time it attacks, chances are the plant will get sick again,” Velasquez says. “Unfortunately for plants, they are stuck with the defenses they are born with, be it thorns, an almost impenetrable bark, antimicrobial compounds, or anything else.

“So, we wanted to know how a plant would fare against a bacterium it might have never encountered before and see what types of resistance strategies it would deploy.”

Velasquez and Oney got their hands on 1,041 natural varieties of Arabidopsis – basically the lab guinea pig of plants – and pit them against the bacterium Pseudomonas syringae, a major foe of tomatoes.

“We subjected the plants to a severe test, more extreme then they probably face in the wild,” Velasquez says. “We dipped each plant in a solution full of very high levels of bacteria, and the pots were placed in humid environments that made the plants susceptible to disease.”

Only 14 plant varieties out of over 1,000 resisted the bacterium, and the researchers observed four previously known defense strategies at work, in addition to another strategy they had not encountered before.

“One of the known mechanisms was a surface-mediated resistance mechanism designed to prevent bacteria from invading. But if the bacteria made it through to the inside of the leaves, the plant was doomed.”

The other defenses were activated once the bacterium made it in. They ranged from producing high levels of a defense hormone, a derivative of which is found in aspirin, to making reactive oxygen species, the same molecules that, in humans, might cause aging (and that we combat with antioxidants).

There was even a peculiar strategy that activates a programmed cell death that wipes out infected plant cells, to prevent the bacteria from invading the rest of the plant.

“We also observed three plants that showed some enhanced resistance, but we don’t know how that worked. We are currently stuck with the knowledge that we have.”

Taking it to the world

Velasquez notes that research such as his will lay the groundwork for studying disease in major food crops.

“Arabidopsis is easy to work with. You can hold tens of thousands of seeds in one handful, and they grow fast, which makes it simple to do experiments quickly. But Arabidopsis has no economic value.”

“Still, we have to start somewhere. It’s tough to study diseases in agricultural crops. They grow much slower. And, we can’t get our hands on as many variants as Arabidopsis, because, over time, plant breeders have selected only a small subset from each species to be consumed as food.”

Take potatoes, Velasquez says. Some wild varieties have no tubers, others would send you to the hospital for poisoning, and others are too small to harvest and make a profit on.

“Yet, they’re all distinct from the limited number of potato varieties that we know well and that we farm. But what if some wild potatoes have natural defenses that are absent in crop potatoes? We’d miss out on discovering those if we only studied the crops that we know.”

Ultimately, André muses, understanding the basic mechanisms of how plants resist disease is a way forward to reduce crop sickness.

“As our knowledge base increases, we can perhaps identify wild potatoes or tomatoes, or any other plant of interest, with specific defenses and cross them with their cultivated relatives. We could even make plants that are more resistant to specific diseases.”

“We are still scratching the surface of what is possible,” Velasquez adds. “The number of plants, bacteria, viruses, and the interplay between them… there is a stunning variety of things going on in nature.”